1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device and a fabricating method thereof having an image sensing function adapted for scanning a document and image and touch-inputting, and a method of sensing an image by using the liquid crystal display device and the associated fabricating method.
2. Description of the Related Art
A liquid crystal display device controls light transmittance of liquid crystal using an electric field in order to display a picture. To this end, the liquid crystal display device includes a liquid crystal display panel having liquid crystal cells arranged in a matrix, and a driving circuit for driving the liquid crystal display panel.
The liquid crystal display panel may include a thin film transistor array substrate and a color filter array substrate opposed to each other, liquid crystal injected between the two substrates, and may include a spacer to maintain cell gap between the two substrates.
The thin film transistor array substrate includes gate lines, data lines, thin film transistors as switching devices at each intersection between the gate lines and the data lines, pixel electrodes formed for each liquid crystal cell and connected to the thin film transistor, and alignment films. The gate lines and the data lines receive signals from the driving circuits via each pad portion. The thin film transistor applies a pixel signal fed to the data line to the pixel electrode in response to a scanning signal fed to the gate line.
The color filter array substrate includes color filters for each liquid crystal cell, black matrices between the color filters, common electrodes for commonly applying reference voltages to the liquid crystal cells, and an alignment film.
The liquid crystal display panel is completed by preparing the array substrate and the color filter substrate individually, joining them, and then injecting liquid crystal between the substrates. The substrates are then sealed with an appropriate sealant.
FIG. 1 is a plan view illustrating a thin film transistor array substrate of a related art liquid crystal display device, and FIG. 2 is a sectional view of the thin film transistor array substrate taken along the line I-I′ of FIG. 1.
Referring to FIG. 1 and FIG. 2, the thin film transistor array substrate includes a gate line 2 and a data line 4 provided on a lower substrate 42 such that they intersect each other with a gate insulating film 44 therebetween, a thin film transistor 6 (hereinafter, referred to as “TFT”) provided at each intersection, and a pixel electrode 18 provided as shown. Further, the thin film transistor array substrate includes a storage capacitor 20 provided at an overlapped portion between the pixel electrode 18 and the gate line 2.
The TFT 6 includes a gate electrode 8 connected to the gate line 2, a source electrode 10 connected to the data line 4, a drain electrode 12 connected to the pixel electrode 18, and an active layer 14 overlapping with the gate electrode 8 and defining a channel between the source electrode 10 and the drain electrode 12. An ohmic contact layer 48 for making an ohmic contact with the data line 4, the source electrode 10 and the drain electrode 12 is further formed on the active layer 14. The active layer 14 and the ohmic contact layer 48 form a semiconductor 45.
The TFT 6 allows a pixel voltage signal applied to the data line 4 to be charged into the pixel electrode 18 and kept in response to a gate signal applied to the gate line 2.
The pixel electrode 18 is connected to the drain 12 of the TFT 6 by a contact hole 16 through a protective film 50. The pixel electrode 18 contributes to a potential difference with respect to a common electrode provided at an upper substrate (not shown). This potential difference rotates liquid crystal molecules positioned between the thin film transistor array substrate and the upper substrate because of dielectric anisotropy and transmits light via the pixel electrode 18 from a light source (not shown) toward the upper substrate.
The storage capacitor 20 is formed by the overlap of the gate line 2 and the pixel electrode 18. A gate insulating film 44 and a protective film 50 are positioned between the gate line 2 and the pixel electrode 18. The storage capacitor 20 allows a pixel voltage charged in the pixel electrode 18 to be stably maintained until the next pixel voltage is charged.
The related art liquid crystal display device only has display capability but does not have sensing capability or external image display capability.
FIG. 3 is a cross section showing a related art image sensing device. The TFT structure of related art FIG. 1 and FIG. 2 is given the same reference numbers as in related art FIG. 3.
Referring to FIG. 3, an image sensing device includes a photo TFT 40, a storage capacitor 80 connected to both the photo TFT 40 and a switching TFT 6 located opposite the photo TFT 40 and having a storage capacitor 80 therebetween.
The photo TFT 40 includes a gate electrode 8 formed on the substrate 42, an active layer 14 overlapping the gate electrode 8 having the gate insulating film 44 therebetween, a source electrode 60 electrically connected to the active layer 14, and a drain electrode 62 opposed to the source electrode 60. The active layer 14 overlaps with the source electrode 60 and the drain electrode 62, and further includes a channel portion between the source electrode 60 and the drain electrode 62. The ohmic contact layer 48, for ohmic contact with the source electrode 60 and the drain electrode 62, is further formed on the active layer 14. The photo TFT 40 senses light transmitted by an image such as a document or a fingerprint.
The storage capacitor 80 overlaps with a lower storage electrode 72 where the lower storage electrode 72 is connected to the gate electrode 8 of the photo TFT 40, with the insulating film 44 therebetween, and includes an upper storage electrode 74 connected to the drain electrode 62 of the photo TFT 40. The storage capacitor 80 stores an electric charge by a photo current generated at the photo TFT 40.
The switching TFT 6 includes a gate electrode 8 formed on the substrate 42, a source electrode 10 connected to the upper storage electrode 74, a drain electrode 12 opposite the source electrode 10, and an active layer 14 overlapping the gate electrode 8 with a channel between the source electrode 10 and the drain electrode 12. The active layer 14 overlaps with the source electrode 10 and the drain electrode 12, and further includes a channel portion between the source electrode 10 and the drain electrode 12.
An ohmic contact layer 48 for ohmic contact with the source electrode 10 and the drain electrode 12 is further formed on the active layer 14.
Driving of the image sensing device with the above structure will be briefly described. For example, approximately 10V is applied to the source electrode 60 of the photo TFT 40, and approximately −5V of an inverse bias voltage is applied to the gate electrode 8. When light is sensed by the active layer 14, a photo current path in the drain electrode 62 and source electrode 60 channel is generated. The photo current path flows from the drain electrode 62 into the upper storage electrode 74, and the lower storage electrode 72 is connected to the gate electrode 8 of the photo TFT 40, so that an electric charge generated by the photo current is charged into the storage capacitor 80. The electric charge charged into the storage capacitor 80 is transmitted into the switching TFT 6 to create an image sensed by the photo TFT 40.
Accordingly, a related art liquid crystal display device has only display capability, and a related art image sensing device only has image sensing capability.